1. Field of the Invention
The present invention relates to a device for detecting pulse waves and, more particularly, to a pulse wave detection device which detects pulse waves by transmitting ultrasound to an artery and by receiving reflected ultrasound from the artery.
2. Description of the Related Art
Detection of pulse waves of blood flowing in arteries is being widely performed in medical and health care environments. For such pulse wave detection, the method of automatically and electronically determining the pulse rate by using a pulse wave detection device is being widely practiced, as is the method of detecting pulse waves by palpation for a predetermined time period to determine the pulse rate.
Known devices capable of determining the pulse rate by electronically detecting pulse waves are, for example, a device having a piezoelectric element used as a sensor to determine the pulse rate in such a manner that the piezoelectric element is positioned on an artery to detect changes in the pressure of the outermost skin layer (displacements of the skin layer due to the pressure) resulting from changes in the pressure in the artery, and a device utilizing ultrasound to determine the pulse rate.
Pulse wave detection devices utilizing ultrasound include those utilizing the Doppler effect of a flow of blood, e.g., one disclosed in Japanese Patent Application Laid-Open No. Hei 1-214335 and one disclosed in U.S. Pat. No. 4,086,916.
FIGS. 9(a) and 9(b) are diagrams showing changes in the frequency of ultrasound according to the Doppler effect. When ultrasound, such as shown in FIG. 9(a), having a frequency f0, is emitted from a body surface to an artery, the emitted ultrasound is reflected by blood flowing in the artery. The reflected sound is received by a receiving element to detect changes in the frequency of the reflected sound. That is, during a heart contraction period, the speed at which blood flows in the artery becomes high and the frequency of the received sound, indicated by f1, becomes higher (in region A) by the Doppler effect, as shown in FIG. 9(b). Conversely, during a heart relaxation period, the blood flowing speed is low and the frequency of the received sound is low (in region B) relative to that in the region A.
As described above, a bloodstream in an artery, the speed of which changes according to pulsation of the heart, is irradiated with ultrasound, and pulse waves are detected by detecting changes in the frequency of reflected ultrasound, thus enabling determination of the pulse rate, the speed at which blood flows, etc.
To detect pulse waves from an artery at a wrist by the above-described conventional method, a sensor is ordinarily positioned on the radial artery because the existence of the radial artery is generally easily recognizable; the position of the radial artery can be easily determined; and the level of a pulse wave signal detected therefrom is comparatively high.
However, the radial artery on which the sensor is placed is located close to a wrist cord. Therefore, there is a possibility of the sensor being lifted or shifted from the suitable position by a movement of the hand to cause noise, which considerably affects the detection of pulse waves. For this reason, a pulse wave detection error or failure can occur comparatively and easily.
Also, while the facility with which the sensor is positioned is influenced by the shape and the size of the sensor, it is difficult to position the sensor on the some people""s wrists. Accordingly, the pulse wave detection performance depends on individual differences of wrists.
In view of the above-described problems of the conventional art, an object of the present invention is to provide a pulse wave detection device capable of accurately detecting pulse waves without being influenced by noise caused by a movement of the hand or the like.
Another object of the present invention is to provide a pulse wave detection device capable of performing pulse wave detection with reliability regardless individual differences in wrists.
To achieve the above-described objects, according to one aspect of the present invention, there is provided a device for detecting pulse waves, comprising a first sensor having first emitting means for emitting ultrasound toward the radial artery and first receiving means for receiving ultrasound emitted from the first emitting means and reflected by blood flowing in the radial artery, a second sensor having second emitting means for emitting ultrasound toward the ulnar artery and second receiving means for receiving ultrasound emitted from the second emitting means and reflected by blood flowing in the ulnar artery, pulse wave information acquisition means for acquiring pulse wave information on pulse waves from the ultrasound received by one of the first receiving means and the second receiving means, and output means for outputting the pulse wave information acquired by the pulse wave information acquisition means.
Thus, two sensors: the first sensor and the second sensor are provided. Therefore, even if the operation of one of the first and second sensors results in a detection error or detection failure, detection can be performed by using the other of the first and second sensors. For example, if the first sensor is unable to perform detection, the second sensor can be operated to perform detection, thus reducing occurrence of a detection error or failure.
Storage means for storing obtained pulse wave information may be provided. Pulse wave information stored in the storage means can be output afterwards. If pulse wave information obtained as a result of detection during a predetermined period of time is stored, the information can be output to, for example, an external apparatus for a medical diagnosis or the like to be utilized for a medical diagnosis of a user""s daily condition.
In the pulse wave detection device according to the present invention, effectiveness determination means may be provided to determine whether the ultrasound received by one of the first receiving means and the second receiving means is effective in detecting pulse wave information. The pulse wave information acquisition means acquires pulse wave information on pulse waves from the ultrasound recognized as effective by the effectiveness determination means. The effectiveness determination means determines the effectiveness of the pulse wave information signals to enable control for selectively acquiring pulse wave information from one of the first and second sensors, thereby reducing occurrence of a detection error or failure.
In the pulse wave detection device according to the present invention, switching means for selectively driving the first sensor or the second sensor may be provided. The switching means selects driving of one of the first and second sensors if it is determined that the ultrasound received by one of the first receiving means and the second receiving means during driving is ineffective. Also, in the pulse wave detection device according to the present invention, the switching means may select driving of the first sensor after a lapse of a predetermined period of time from the time when the switching means selected driving of the second sensor. If this selective drive enabled by the switching means is performed, there is no need to emit ultrasound so that both the first and second sensors always receive ultrasound. Ultrasound may be emitted to be received by only one of the two sensors selectively used, and the consumption of electricity can be reduced.
In the pulse wave detection device according to the present invention, indication means may also be provided. If the effectiveness determination means determines that both the ultrasounds received by the first receiving means and the second receiving means are ineffective, the indication means indicates this condition. This indication means enables the user using the pulse wave detection device to know whether pulse waves are being accurately detected. If the pulse wave detection device is unable to detect pulse waves due to, for example, a failure to maintain the device in the correct position, the user may press a reset button or turn off the power and then turn on the power again, thereby accurately redoing pulse wave detection.
The pulse wave detection device according to the present invention comprises a first sensor having a first transmitter to transmit ultrasound toward the radial artery, and a first receiver to receive ultrasound transmitted from said first transmitter and reflected by blood flowing in the radial artery, a second sensor having a second transmitter to transmit ultrasound toward the ulnar artery, and a second receiver to receive ultrasound transmitted from said second transmitter and reflected by blood flowing in the ulnar artery, a pulse wave information acquisition circuit to acquire pulse wave information on pulse waves from the ultrasound received by one of said first receiver and said second receiver, and an output circuit to output the pulse wave information acquired by said pulse wave information acquisition circuit.